Control valves (e.g., sliding stem valves, rotary valves, axial flow valves, etc.) are commonly used in process control systems to control the flow of process fluids. A control valve typically includes an actuator (e.g., a pneumatic actuator, hydraulic actuator, etc.) that automates operation of the control valve. Typical actuators are coupled to an exterior surface of the valve body (e.g. the valve bonnet) of the control valve. Some applications, particularly in process control systems having larger diameter sliding stem valves or rotary valves, may have performance limitations because larger sized sliding stem or rotary valves often require long-stroke actuators to actuate a flow control member of the valve. Such long-stroke actuators increase the weight and the dimensional envelope of the control valve assembly. Thus, in some applications, large-sized sliding stem or rotary valves may not be practical.
Axial or in-line flow control valves are an alternative to sliding stem and rotary control valves. Axial flow valves have a flow path or passageway that is substantially parallel to the fluid flow path to minimize turbulent flow through the valve body. Axial flow control valves typically include an actuator mounted to an exterior surface of the valve body and oriented substantially perpendicular to the fluid flow path. The actuator is operatively coupled to a flow control member of the valve and moves the flow control member between an open position and a closed position to allow or prevent the flow of fluid through the valve. Some known axial flow control valves include an externally mounted rack-on-rack (i.e. as opposed to a rack-and-pinion) actuators to actuate a flow control member within the valve body relative to a seat ring to control fluid flow through the valve body. Mokveld Valves BV of Houston, Tex. offers such a known axial flow control valve.
However, externally mounted actuators may be susceptible to unwanted leakage past the valve body. More specifically, such an externally mounted actuator configuration requires use of packing to seal against a valve stem or shaft that extends outside the valve body to couple to the actuator. Such packing can fail (e.g., due to wear) and result in leakage of the fluid outside the valve body and into the environment surrounding the valve and add additional costs to the valve, and/or add additional maintenance requirements and costs. Because the actuator is substantially perpendicular to the flow path of the fluid in such an arrangement, the externally mounted actuator configuration may require additional space which, in practice, is often very limited.
Additionally, axial flow control valves and, particularly larger sized axial flow control valves, having externally mounted actuators require more precise alignment to prevent undesired leakage between the valve body and the actuator. Such precise alignment and accurate, tighter tolerances typically require more precise manufacturing and machining. Therefore, such known control valves are complex and, thus, more expensive to manufacture because they require additional machining and may require additional maintenance.